Such pumps generally have a head unit containing the pump pares proper, a drive unit and a motor unit, each provided with a separate case manufactured from aluminum sand castings, aluminum permanent-mold castings or aluminum diecastings. While a disadvantage of the cases manufactured from aluminum die-castings lies particularly in the high tool costs, those made from aluminum sand castings and aluminum permanent-mold castings display large work tolerances, a tendency towards cavitation, and irregular, rough surfaces. These known pumps are therefore difficult to clean, particularly since dirt is apt to lodge at the transition points between the individual subassemblies and is hard to remove.